Soil nitrification and foliar δ¹⁵N declined with stand age in trembling aspen and jack pine forests in northern Alberta, Canada

Background and aims: Understanding changes in soil N cycling with stand development is critical for forest management as tree growth is affected by soil N availability. The aim of this study was to evaluate the changes in soil N availability and loss with stand development in trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.) in northeastern Alberta, Canada. Methods: Soil inorganic N availability (measured as N supply rate) and foliar N chemistry (N concentration and δ¹⁵N) in trembling aspen stands ranged from 52 to 70 years old (n = 7) and jack pine stands 43 to 78 years old (n = 8) were investigated in 2008 and 2009. The relationships among the ratios of NO₃ ^--N to total inorganic N (NO₃ ^--N/TIN), foliar N concentration, and foliar δ¹⁵N with stand age were also explored by regression analyses. Results: Total inorganic N supply rates did not systematically change with stand age across stand types, soil layers and measurement periods; whereas NO₃ ^--N/TIN showed a decreasing tendency with stand age, suggesting that nitrification and associated N loss potential became smaller in older stands with greater limitation in soil N availability. Foliar δ¹⁵N decreased with stand age from −1.7 to −4.7‰ for aspen and from −4.1 to −7.1‰ for jack pine, and there were positive correlations between foliar δ¹⁵N and soil NO₃ ^--N/TIN, suggesting that decreased soil N loss led to less ¹⁵N-depletion in the inorganic N available for tree uptake in older stands. However, foliar N concentration did not significantly change with stand age, suggesting that there were other N sources such as organic N in the forest floor, in addition to the inorganic N, available for plant uptake. Conclusions: Our results suggest that soil inorganic N availability became more limited as stand age increased. In addition, the ratio of NO₃ ^--N/TIN and its relationship with foliar δ¹⁵N indicated decreased soil N loss potential and shifted N sources with stand age in boreal forests that are typically N-limited. Our study demonstrated that declining nitrification with increasing stand age might be one of the mechanisms mediating N-limitation in the studied boreal forests.